This invention relates to measuring thickness of sheet material without contacting the material and is particularly directed to measuring material thickness in a blown film sheet and bag machine.
Plastic sheet and bags are often manufactured in blown film machines. A typical blown film machine has a base with a source of heated plastic and an air supply. The plastic is extruded upward through an annular die in a cylindrical configuration. Air is blown into the interior of the cylinder expanding the plastic to the desired size. The upwardly moving plastic sleeve or bubble freezes or hardens a short distance above the die and is lifted and flattened or collapsed by a mechanism at the upper end of the bubble producing a flattened double layer sleeve for subsequent operation. A typical machine can produce plastic in thicknesses up to 50 to 100 mils with bubbles in the order of 12 feet tall and 2 to 8 feet in diameter.
The air blown into the interior of the bubble is heated, and the thickness of the sheet is controlled by controlling the air temperature and the air pressure. It is preferred to have the extruder rotate slowly, typically one revolution per minute, for improved uniformity of thickness.
Two configurations for measuring thickness are presently being used. One system utilizes a beta radiation source which rests against the plastic sleeve directing beta rays through the sleeve and measuring the scattered radiation from the air on the other side of the sleeve. The beta source must maintain continuous contact with the sleeve and produces a dimple in the bubble. There are a number of disadvantages of this system. The measurement is a function of the air density and air temperature which introduces considerable error. The measurement must be made by contact with the solid plastic above the frost line and it is difficult to relate the measured location to a location on the extruder die. This is significant since the thickness varies around the circumference of the die and it is desirable for the operator to know the exact location on the annular die which needs adjusting. Also, the contact of the source with the material produces distortion and friction problems.
In the second system, thickness is measured at the flattened double layer either by direct contact or by the beta gauge previously described. The major problem with this type of measurement resides in the fact that the measurement is made too far from the extruder die prohibiting close control of film thickness.
It is an object of the present invention to provide a new and improved apparatus and method for measuring sheet thickness of a blown film machine. A particular object is to provide for measurement without contact with the sheet material and to provide measurement immediately adjacent the extruder die. It is a further object of the invention to provide a new and improved method and apparatus for measurement of thickness suitable for use with various sheet type materials which are capable of being penetrated by x-rays, such as plastics, rubbers, gums and metals.